Optical switches have been developed using guided wave devices or free-space mechanical devices. Guided wave devices use waveguides, whereas free-space devices use optical beams in free space with movable optical elements such as mirrors or lenses.
Guided wave devices typically divert light from one arm of the device into the other by changing the refractive index of one of the arms of the device. This is typically done using electrical, thermal, or some other actuating mechanism.
The free-space approach has an advantage over the guided-wave approach in some applications. It has very low cross talk because the waveguides are physically isolated from one another and coupling cannot occur. The principal source of cross talk in this approach is scattering off the movable optical element. In addition, free-space devices are wavelength-independent and often temperature-independent.
Existing designs employ mirrors positioned at the intersection of input fibers and output fibers. Due to the spreading of the light beam as it leaves the waveguide and travels toward the mirror large mirrors are used that require mounting and angular placement accuracy. There can be significant difficulty in actuating such a relatively large structure quickly and accurately at the switching speeds required for optical communication systems.
Thus, a need exists for an optical switch having the advantages of the free-space approach, without the disadvantage of existing designs.
The present invention relates generally to the field of optical MEMS (micro-electro-mechanical system) and more specifically to the use of fabrication techniques used in making micromechanical devices to fabricate high speed optical MEMS for optical communication networks. The method employs the use of a removable layer that is formed between an optical waveguide and a movable switch element that has been formed over the removable layer and the waveguide. The removable or sacrificial layer is preferably formed using a conformal material such as parylene (poly-para-xylene).
A preferred embodiment of the invention can use a silica substrate with optical waveguides formed therein as the initial structure in the manufacture of the optical MEMS. After formation of a trench in the substrate to define a gap between waveguide elements, a first mask is used to define the routing wire geometry on the upper surface of the substrate. The trench can have a width of about 3 to 20 xcexcm. Subsequently the removable layer is formed followed by the use of a second mask for fabrication of a switch element layer.
After patterning of the switch element layer, a spacer layer, preferably a photoresist layer, is spun on the surface and patterned using a third mask. A metallization layer, preferably an evaporated layer of copper, is deposited and a further photoresist layer is formed using a two mask exposure sequence. This defines a mold for fabrication of a plating layer. In a preferred embodiment of the invention, nickel is electroplated into the mold to form an integral electrode structure.
The removable layer is then preferentially etched to release the switch element which has been fabricated with a spring that supports the switch relative to the substrate. The characteristics of the spring define the speed and pull-up voltage of the switch element.
The electrodes are used with an overlying actuating electrode structure to actuate movement of the switch element between states. A preferred embodiment of the invention uses a reflective element or mirror that is moved from a first position, in which light from a first waveguide is reflected by the mirror into a second waveguide, to a second position in which the mirror is translated vertically to permit light to pass through the gap on a linear optical path into a third optical fiber that is aligned along a single axis with the first fiber. The waveguides and/or the trench can be filled with air or in another embodiment can be filled with a fluid. Further details regarding the use index matching fluids are described in International Application No. PCT/US99/24591 filed on Oct. 20, 1999, the entire contents of which is incorporated herein by reference.
Another preferred embodiment of the invention involves the fabrication of an array of switches on a single substrate that can serve as a monolithic array, or alternatively, the substrate can be diced to provide separate switches or arrays of switches.